Various publications, including patents, published applications, technical articles and scholarly articles are cited throughout the specification. Each of these cited publications is incorporated by reference herein, in its entirety, for all that it teaches.
Ventricular repolarization time is determined by transmembrane action potential duration (APD) of the ventricular myocardium or the QT interval on the body surface electrocardiogram (ECG). Delayed ventricular repolarization that manifests as QT interval prolongation on the ECG is associated with the development of an atypical form of polymorphic ventricular tachycardia termed torsade de pointes (TdP) that can result in recurrent fainting and sudden death in humans. An increasing number of medications are found to prolong ventricular repolarization, leading to QT prolongation. Some of these medicines are developed purposefully to prolong cardiac APD for the treatment of cardiac arrhythmias such as atrial fibrillation and monomorphic ventricular tachycardia. These medications are called antiarrhythmic drugs, and include sotalol, dofetilide and amiodarone. Many such drugs are administered in suboptimal doses to avoid TdP. However, suboptimal dosing of these drugs in humans in order to avoid TdP greatly attenuates their efficacy in the inhibition of cardiac arrhythmias. On the other hand, many non-cardiac agents, such as cisapride, terfenadine, erythromycin and sparfloxacin, have been removed from the market or relabeled for restricted use because of their proarrhythmic potential. Recent regulatory guidelines recommend preclinical assessment of potential new drugs for QT prolongation and the resultant risk of TdP in humans.
Several technologies and methods are currently available for preclinical testing of compounds for their potential to cause TdP. One such method is to measure a drug's effect on the ionic current in stable cell lines that express the hERG channel. In humans, the ether-a-gogo related gene (hERG) potassium (K+) channels play a role in the control of action potential duration in cardiac cells. In the cell, this K+ channel underlies the cardiac repolarizing K+ current Ikr, returning the cell to its resting state. (Sanguinetti, M. C., et al. Cell 81:299-307 (1995)). Because blockage of the hERG channel generally results in prolongation of the action potential, as well as the prolongation of the QT interval, the effect of a drug on the Ikr current is thought to be correlated with the drug's potential to cause TdP. (Joshi A K et al., J. Electrocardiol. 2004; 34(supplement): 7-14).
This screening method, however, suffers from several major drawbacks. The drawbacks include the fact that this method is not well suited for high throughput screening, and that TdP risk is not proportional to the potency of a compound to inhibit hERG current. In other words, this method has a fairly high potential to produce false negatives and positives. A false negative may result if the hERG channel is not the K+ channel target of the compound. False positives may result where the blockage of the hERG channel does not directly correlate with a prolonged QT interval. In addition to the Ikr current, there are other ventricular membrane currents which may be affected by a drug. The ventricular action potential duration, which determines the QT interval, is the consequence of a dynamic balance of multiple membrane currents. Thus, testing only the effect of a drug on the Ikr current, may overlook the drug's effect on other membrane currents such as those produced by Calcium or Sodium ion channels, thereby blurring the picture of the drug's effect on the QT interval. The typical examples are verapamil and amiodarone. Verapamil is a potent hERG current inhibitor, yet is not associated with significant QT prolongation and is free of TdP risk in humans (Yang T et al., J. Cardiovasc. Pharmacol. 2001; 38: 737-744). Similarly, amiodarone inhibits hERG current at fairly low concentrations and significantly prolongs the QT interval, although it rarely causes TdP in humans (Mattioni T A et al. Ann. Intern. Med. 1989; 111: 574-580).
A second screening method involves the direct measurement of action potential duration in isolated ventricular Purkinje fibers or ventricular myocardium. The rationale behind this method is the expectation that a compound would be likely to cause TdP if it can be shown to increase action potential duration. (Champeroux, P., et al. Br. J. Pharmacol. 144:376-85 (2005)). This screening method, however, is not a very sensitive assay, and thus is generally done in conjunction with another screen such as hERG inhibition.
This screening method also suffers from an additional drawback in that it has a high potential to cause false positive or negative results. Examples of false positives include drugs such as amiodarone, which may cause a prolonged action potential duration or QT interval (van Opstal J M et al., Circulation 2001; 104:2722-2727), yet not be potent inducers of TdP. Examples of false negatives include the drugs bepridil and terfenadine, which are known to cause TdP in humans, but have not been shown to produce a significant increase in the action potential duration of the Purkinje fiber (Champeroux et al., Br. J Pharmacol. 2005; 144:376-385). Experimental data show that bepridil fails to produce a significant increase in the QT interval (Coumel et al., Fundam. Clin. Pharmacol. 1993; 7: 61-68), although it is associated with a significant risk of TdP and sudden death (Coumel P et al., Am. J. Cardiol 1992; 69: 75D-78D). Thus, the lack of sensitivity of this assay may result in a compound's true propensity to cause TdP to go unrecognized in preclinical screening.
A third screening method involves the measurement of the QT interval or the duration of monphasic action potential in the isolated, Langendoff-perfused heart. (Hondeghem, L. M., et al., Circulation, 2001; 103:2004-13). This method suffers from a significant drawback as well. The drawback is that the electrical stability of the perfused heart preparations is of relatively short duration, on the magnitude of less than two hours. The instability of the preparations may thus result in false negative or false positive results.
Interestingly, the QT interval (or ventricular APD) seems to be positively proportional to body mass among various species under physiological conditions, ranging from tens of milliseconds in mice to hundreds of milliseconds in large animals as shown in FIG. 1. APD prolongation in small animals to an extent may lead to occurrence of early after depolarization (EAD) capable of initiating TdP, but in larger species, the same or longer APDs can be physiological. For example, the physiological QT interval in the cow is approximately 400 ms, but the same length of QT would be likely associated with a high risk of TdP in the rabbit. This raises a compelling question: what is the mechanism that dictates if a prolonged QT interval is physiological or pathophysiological? Apparently, there is a mechanism for stabilizing cell membrane potentials in the large species, so that a normal heart rhythm can be maintained under a relatively longer QT interval.
TdP is triggered by ventricular action potential EAD at repolarization phase 2 or phase 3. In other words, EAD as the trigger plays a central role in the development of TdP, and its occurrence in ventricular action potentials could serve as a marker to determine if a prolonged QT interval is physiological or pathophysiological. Since the L-type calcium channel current (ICa) is the primary charge carrier for phase 2 EADs under delayed ventricular repolarization (Clancy & Rudy., Nature 1999; 400: 566-569; Viswanathan & Rudy., Cardiovasc Res. 1999; 42: 530-542), its availability after the initial activation for the development of EAD during repolarization phase 2 is critical for the development of TdP.
The foregoing discussion indicates that there exists a need for an economical and more accurate procedure to screen compounds for their capacity to cause a prolonged QT interval and potentially life-threatening cardiac arrhythmias such as TdP. Similarly, there exists a need for an economical and more accurate procedure to screen compounds for their antiarrhythmic potential. Such accurate procedures would have substantial implications for the pharmaceutical industry's development of drugs to treat cardiac arrhythmias without the adverse side effect of causing an arrhythmia or other adverse condition, as well as for the development of non-cardiac therapeutic agents. It is desirable that such a method significantly reduce, and even more preferably eliminate, false positive and false negative results. It is also desirable that such a method be amenable to high throughput screening. The present invention is directed to a more accurate procedure, and addresses these long-felt needs.